Last year I switched all of my fermenters to the 6 gallon model of these. They're incredibly thick, cheap, light, strong, and durable. It's basically a win/win.
For the bottom stacked containers I drill a hole in the recess that houses the lidded opening, then threaded in a brass elbow with an o-ring on either side, finished with a nut. On the tops of the stacks, I cut out the center hole from the lid with a standard rubber stopper fitting quite nicely in the 1" or so channel.
Does this make sense or help at all?